Glioblastoma (GBM) is an intractable disease with high mortality. In the U.S, the incidence of GBM is approximately 9,600 new cases/year. Even with current standard-of-care, surgical debulking followed by concurrent radiotherapy and chemotherapy with temozolomide, survival remains modest. There is a clear, major unmet medical need for novel approaches to treat patients with GBM. The goal of this four-year application is to evaluate the innovative approach of targeted prodrug therapy for GBM. In this investigator-initiated Phase 2 clinical study, the applicant will test a vascular-targeted prodrug, G-202 in patients with recurrent GBM. G-202 is a thapsigargin analog (12ADT) coupled with a masking/targeting peptide that renders the prodrug inactive and highly soluble (expected to cross blood-brain barrier). This peptide also targets the prodrug to a protein - prostate-specific membrane antigen (PSMA), expressed selectively on the tumor vasculature. On binding PSMA, the prodrug is cleaved and 12ADT is released, which in turn produces endoplasmic reticulum (ER) stress, apoptosis and cell death. The central hypothesis for this proposal is that G-202 will be clinically active against GBM. The preliminary data show that PSMA is selectively and highly expressed in GBM tumor vasculature. Further, patient-derived GBM cell lines are highly sensitive to thapsigargin in vitro. Studies on mouse xenograft models show renal, breast and prostate cancers to be highly sensitive to G-202. The objective of this study is to evaluate the clinical efficacy of G-202 against recurrent GBM and to investigate the effect of 12ADT on preclinical models of GBM. The applicant proposes three specific aims to test the hypothesis and achieve their objective. Aim 1 will determine the efficacy of G-202 in patients with recurrent GBM in a single-arm, open-label, Phase 2 clinical trial. Aim 2 will involve correlative studies - cerebrospinal (CSF) pharmacokinetics to confirm optimal dosing of G-202 and detailed blood/CSF and tissue biomarker analyses in GBM patients treated with G-202. These studies will attempt to identify markers predictive of response to G-202 and will provide guidelines to stratify patients for future trials. Aim 3, using patient-derive GBM cell lines, will seek to identify molecular predictors of responsiveness to 12ADT, the thapsigargin analog in G-202. In preliminary studies, the applicant found a differential sensitiviy of different GBM lines to thapsigargin that correlated with expression levels of GRP78, the master regulator of the ER stress response.